1. Field of the Invention
The present invention concerns a device for regulating the transfer of a load suspended by cables from a carriage movable in translation in a lifting machine.
2. Description of the Prior Art
Lifting machines such as travelling overhead cranes and gantry cranes comprise a carriage movable in translation by at least one horizontal travel motor connected to a variable speed drive and at least one lifting motor associated with cables and with a load suspension device to lift or lower the load. The load is displaced from an initial position to a final or target position by the combination of horizontal and vertical movements.
Due in particular to the accelerations imparted to it, the suspended load acquires in the direction of horizontal displacement a weakly damped pendular motion, called sway hereinafter, which must be controlled to transfer the load gently and safely. In the specific case where the machine is a container gantry crane, the precision with which the container must be put down is on the order of 5 cm and this requires the swaying movement to be eliminated.
The patent FR-2 571 867 describes damping the oscillation of a suspended load by dynamic control of the acceleration of the carriage which entails adding to a signal controlling the carriage travel motors a corrector signal determined from the angular speed of the pendulum. The system described is not able to control the position of the carriage or to offer the operator a choice of modes of operation.
The patent WO-91 14 644 describes a method of transferring a suspended load in which oscillation of the load is reduced by predictive control using a mathematical model while positioning the load. The drawback of a method of this kind is that it does not have the required performance in all cases, in particular for displacements at low speed or over short distances. Also, predictive processing requires that the path of the load be determined and so rules out dissociation of load lifting processing and carriage displacement processing. The only control mode that can be used is an automatic cycle.